While there is general consensus that interferon-gamma (IFN-gamma) is vital to the clearance of Listeria monocytogenes, the cellular source of IFN-gamma has been controversial. Some investigators have concluded that NK cells are responsible, while others have implicated T cells. Recently, we have identified a novel cell type that secretes large amounts of IFN-gamma and exists in the spleen and other organs of normal mice. Because these unique cells lyse targets directly, activate naive T cells in vitro and in vivo, and express both the natural killer cell marker NK1.1 and the dendritic cell marker CD11c, we have labeled them as natural killer dendritic cells (NKDC). A few other investigators have recognized NKDC-like cells in rodents and humans. We have preliminary data that splenic NKDC produce IFN-gamma during Listeria infection in mice. Therefore, we hypothesize that NKDC are critical to the immune response against Listeria. While the lytic and antigen-presenting abilities of NKDC may be important, we have chosen to focus exclusively on their IFN-gamma production for this 2-year application. We propose to assess the biologic significance of IFN-gamma production by NKDC during Listeria infection in mice and dissect the mechanisms by which it is regulated. In Aim 1, we will determine the extent of NKDC involvement in IFN-gamma production during Listeria infection. To exclude the possibility that other cells make IFN-gamma in vivo, but are unable to do so once they are removed from their local environment in the spleen, we will make use of a recently described technique of in vivo intracellular cytokine analysis of splenocytes. In Aim 2, we will establish the mechanism of IFN-gamma production by NKDC during Listeria infection. Specifically, we will test IL-12, IL-18, and Toll-like receptor 9 as we have found these to regulate NKDC IFN-gamma production in response to bacterial CpG. In Aim 3, we will determine whether NKDC production of IFN-gamma is sufficient to induce an immune response against Listeria. We will adoptively transfer NKDC from wild-type mice into IFN-gamma deficient mice and then assess their response to Listeria infection. NKDC will be compared to NK cells and other splenic cell subsets that make IFN-gamma as identified in Aim 1. Our proposed experiments will begin to define the role of NKDC in the immune response to Listeria, a pathogen that is a cause of human mortality and a potential agent for bioterrorism, and provide a greater understanding of how this novel cell type may participate in other types of immunity. [unreadable] [unreadable] [unreadable]